KABUL (AP) — NATO has raised the number of service members killed in Afghanistan to six, including three Americans.

That makes Monday the deadliest day for the international force in more than two months, underscoring warnings that casualties will increase as more foreign troops stream into the country and step up efforts against the Taliban.

U.S. military spokesman Col. Wayne Shanks says the Americans died when they were caught up in a firefight with militants during an “operational patrol” in southern Afghanistan.

France says one of its sergeants was killed during a clash while patrolling with Afghans in Alasay, a valley largely under insurgent control.

NATO says two more service members were killed — one in eastern Afghanistan and another in the south. But it hasn’t given their nationalities.